Auto merge of #65572 - ecstatic-morse:mir-visitor-break, r=<try>

Return early from a MIR `Visitor`

This allows functions to return a `Result` from the `visit_*` methods on a MIR `Visitor`. Returning an `Err` will stop visitation. This PR is an exploration of an idea I floated [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Early.20return.20from.20a.20MIR.20.60Visitor.60.3F). It shouldn't land as is.

The diff for the `is_min_const_fn`change is easier to read if whitespace is ignored.

r? @nikomatsakis

Author: bors

src/librustc/lib.rs

@@ -50,6 +50,7 @@
 #![feature(thread_local)]
 #![feature(trace_macros)]
 #![feature(trusted_len)]
+#![feature(try_trait)]
 #![feature(vec_remove_item)]
 #![feature(stmt_expr_attributes)]
 #![feature(integer_atomics)]